- Tag results for Roger Federer
The 36-year-old, the oldest No.1 in history, thrashed ailing Bulgarian Grigor Dimitrov 6-2, 6-2 in a one-sided final.
The 36-year-old Swiss earned a return to the top when the rankings are updated Monday, by reaching the semifinals in Rotterdam.
Fourteen years after first grabbing the number one ranking, the 36-year-old Swiss maestro is back on top of the world, the oldest man to achieve such a feat in the sport's history.
Federer surpasses Andre Agassi, who held the top spot aged 33 years and 131 days in 2003, as the oldest man to claim the world number one spot.
It would be Federer's first time back at the summit since October 2012 having first claimed the top spot in February 2004.
The 36-year-old top seed, a two-time champion at the Dutch indoor event, needs to reach the semi-finals to take back the number one spot from Rafael Nadal.
Dimitrov won the opening set against Sugita but fell behind a break in the second before levelling for 5-all.
Federer closed to within 155 points of Nadal after his win last month at the Australian Open against world Cilic.
Roger Federer will target a return to the top of the rankings as the Rotterdam World Tennis tournament starts on Monday.
Rafael Nadal remained in the number one spot in the men's Association of Tennis Professionals world singles rankings.
France are the defending champions and start their campaign at home to the Netherlands while the United States, the record 32-time champions, travel to Serbia.
Roger Federer closed to within 115 points of top-ranked Rafael Nadal of Spain in the Association of Tennis Professionals men's singles rankings after winning the Australian Open.
Roger Federer is showing no signs of slowing down after winning his 20th Grand Slam title at the Australian Open.
The tournament referee made a late call to close the roof and Cilic said it was tough to acclimatize to playing indoors after practicing outdoors earlier.
The Swiss tennis star beat world number three Croatian Marin Cilic in a hard-fought match 6-2, 6-7, 6-3, 3-6, 6-1 at the Rod Laver Arena on Sunday.